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In the Forum: Analog Playback
In the Thread: 7788 tube mic pre design
Post Subject: High transconductance and the Hawaiian archipelago.Posted by Romy the Cat on: 1/13/2008


in my corrector (Dima’s design with R2=50R) I made many experiments adjusting the VR1. Dima proposed 150V on 7788 plate purely arbitrary, what he did it he never ever seen 7788 and it was just a concept RIAA for him. After it was built and I played with it turned out that 150V was a dead right voltage and I do swear that I was able to head what the voltage change to 160V and to 140V. So, you idea to “re-use a low voltage B+ supply”  might not be so economical after all…. BTW, I know that you are creasy person who like to put low bias tube at high-signal input… therefore I  hope you you’re your 7788 with it’s sub 1V of bias input at phono/mic preamp input not at line-level. At line-live that tube shell melt… It should not see more than couple hundreds mV AC on it’s grid…

Jim, I am very confident that you do something very wrong. I have practically no skills in the field of electronics DIY and the damn 7788 gives absolutely no problems to me – that makes this tube absolutely fool-proof even I was able to cook a phonocorrector with it. I mean you with you expertise shell be able to use the 7788 with no aggravations of any kind… Something you do is just wrong or you just have a bad party of the tubes.

BTW, I can testify that I did experience bad 7788. 5-6 year ego I bought on eBay a party of 150 tubes, they were all 7788 Amperex, all pulls and the price “Buy Now” was ridicules – like $40 or something like that. When I got the tubes a half of them were almost back – they were VERY severely burned out. I tested a few dozens of them that had clean glass and the almost all of them has no transconductance.  Among all party I pick 4 of 5 tubes that were more or less useable.  The rest I put back on eBay as pulls and went to somebody in Europe for $350. I do not know, I teach people for year never buy anything from me as whatever I sell is crap that I hate – people just do not listen. Anyhow, this good Amperex that I still have also had no noise problems. So, it is something on your side… You live on a volcanic island, perhaps the screen of the 7788 is too sensitive to those dally harmonic tremors…

The Cat

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